The effects of various precipitation conditions on the formation of stable AgBr crystals have been studied in a balanced double-jet precipitation by Leubner, Wey and Jagannathan and have been disclosed in e.g. Phot. Sc. Eng., Vol. 21 (1977), p. 14, Vol. 23 (1977), p. 248-252 and Vol. 24 (1980), p. 268-272: in J. Imaging Sci., Vol. 34(1990), p. 202-206: in J. Cryst. Growth, Vol. 51 (1981), p. 601-606: and in "Proceedings ICPS Int. Congress of Phot. Science 1986": Progress in Basic Principles of imaging Systems, p. 60.
From these documents the number of nuclei generated during the nucleation stage can be calculated in a semi-empirical way, provided that the values in the reaction vessel of pAg, temperature, concentration of silver complexing reagents and flow rate of the silver and halide containing solutions are controlled during the said stage.
In these well-defined circumstances the crystal size distribution at the end of the precipitation stage is perfectly predictable but an important condition is that during the process of precipitation, proceeding in an aqueous solution of gelatin as protective colloid, two principal stages can be distinctly provided, namely a nucleation stage and a growth stage.
During the nucleation stage no agglomeration may occur so that the number of nuclei formed stays predictable, whereas during the growth stage further control is required in such a way that no renucleation can appear.
Although playing a crucial role in both stages of the precipitation, the role of the protective colloid is sometimes underestimated: said protective colloid has to be considered, not only as a dispersing medium for the fully grown crystals, but particularly as a stabilizing medium for the nuclei. So aggregation of nuclei can be prevented and during the further crystal growth stage the interaction of the protective colloid with the growing crystal surface moderates the crystal growth rate, thereby preventing renucleation as has been suggested by Antoniades and Wey in J. Imaging Sci. and Technol. Vol. 36 (1992), p. 517-524.
From the considerations given above it can be concluded that the protective colloid is determining to a large extent the average crystal diameter and the homogeneity of the crystal size distribution and is offering the possibility to controll the crystal size and the crystal size distribution.
Moreover the protective colloid is also determining the physical properties of the coated film material wherein the silver halide emulsions are incorporated. Properties as e.g. dimensional stability, scratchability, curl, pressure sensitivity and sludge formation after processing are highly dependant on the choice of the protective colloid. A phenomenon like pressure sensitivity may appear as pressure marks, pressure sensitisation or desensitisation, wherein both the protective colloid and the coated matrix have to dissipate the energy developed by the pressure force when the coated layer is dried and deformed afterwards in packaging, before and after exposure and by processing. As adsorption of the protective colloid at the crystal surface occurs development characteristics are further influenced by the said protective colloid.
As has been set forth in EP-Application No. 528 476 a method of preparing a silver halide light-sensitive photographic material incorporating layers of silver halide precipitated in colloidal silica serving as a protective colloid is given. In this document the silver halides are prepared in colloidal silica, leading to emulsion crystals that are stable at the end of the precipitation, but without having a predictable mean crystal diameter and crystal size distribution. These problems have been overcome as has been described in EP-A No. 93202679, filed Sep. 16, 1993, for the preparation of silver bromide and silver bromoiodide crystals. However the problem remains for the preparation of silver halide crystals rich in chloride having a higher solubility product. From a series of experiments it has become clear that an extrapolation to said crystals rich in chloride of the clearly defined circumstances wherein silver bromide or silver bromoiodide crystals can be prepared is impossible and that other ways to reach the objects of this invention, set forth below, should be explored.
"Silica" silver halide crystals rich in chloride are highly preferred in order to prepare very fine, stable grains.
For use in, e.g., daylight materials, such grains having a diameter of less than 0.15 .mu.m, and more preferably of less than 0.10 .mu.m, cannot be prepared at lower temperatures in gelatinous solutions in the reaction vessel, so that grain growth restrainers are used at the required higher temperatures. Moreover grain growth restrainers have a disturbing influence on sensitometric properties of the emulsions coated therefrom.
Fine grains prepared at low temperature in silica sol as a protective colloid may further be used as "feeding reagent" in Ostwald ripening processes in order to grow coarser and/or less soluble crystals.
Moreover exposure light is diffracted less by silver halide crystals rich in chloride, which is in favour of sharpness, as has been illustrated in EP 580 029.
Further a reproducible preparation method for silica silver halide crystals rich in chloride is preferred because of the better solubility of silver chloride versus other silver halides, resulting in a better archivability (due to a more rapid fixation in the fixer by processing). Moreover varying processing conditions, due to, e.g., exhaustion of processing liquids, have less influence on sensitometric properties, obtained in the processing of silver halide photographic materials, coated from emulsion layers, comprising silver halide crystals rich in chloride. Reduced regeneration volumes can further be expected.